One of the problems surrounding the utilization of solar energy on the kilowatt and megawatt level is the problem of concentrating this much energy onto a solar energy receiver. One of the ways of concentrating this much solar energy involves the use of as many as 10,000 individually steerable mirrors in a so-called mirror field covering many acres in which the mirrors redirect sunlight to a solar energy receiver which may be at the top of a mast or tower in the middle of the field, or at any other suitable location such as the side of a hill. Each mirror must take on a different orientation to redirect that portion of the solar energy falling on it to the solar energy receiver. It will be appreciated that even a one degree angular misorientation of an individual mirror may cause the sunlight reflected by it to miss the solar receiver entirely. While it is possible to individually program the driving system for each mirror to correct for the sun's apparent motion, individual programming suffers not only from the complexity of the apparatus needed at each individual mirror but also suffers from its inability to take into account pointing errors due to transient effects such as for instance wind loading, temperature gradients, sway of the power tower, turbulence effects and even ground movement. While it is at least theoretically possible to measure and program in these factors, there is no assurance that the physical systems involved will operate sufficiently in accordance with theory to permit accurate aiming or pointing of the individually steerable mirrors. Moreover, with individual programming it is necessary to continuously or at least on a periodic basis align and calibrate the driving systems for each individual mirror. Even if this is done on a weekly basis, the cost is significant and significant amounts of solar energy may be lost. It is also recognized that individual programing for each of the mirrors in the mirror field need not be done at an individual mirror but may rather be done by a central processor. However, the central processing still suffers from the same difficulties mentioned hereinbefore.
It has also been suggested that a sensor be mounted on an external servo-driven boom at the center of each mirror, with boom alignment the prerequisite for mirror alignment. However each boom presents a substantial lever arm and therefore requires heavy support structure to maintain boom position and to prevent mirror flexure during high winds. Moreover, the boom is external, exposed to the outdoor environment and is fragile and costly.
This invention solves the above problems by placing the entire mechanism behind the mirror and providing each of the mirrors with a central aperture and/or lens to form images of the sun and receiver on surfaces behind the mirror and by aligning the infrared (IR) image of the solar receiver with a reflection of the image of the sun by rotating the mirror about its central aperture. In one embodiment, each mirror is provided with a projection screen type imaging surface in the form of a light weight non-specular light dispersive semi-transparent member on its back side, onto which an image of the sun is formed. In this embodiment this screen takes on the form of a hemisphere with its periphery attached to the back of the main mirror for exceptionally stable support and ease of construction. In this embodiment, a limited aperture reflector is positioned on the back of the main mirror to reflect the sun's image from the projection screen to a combined solar image and receiver image detector, only when the main mirror is pointed in the proper direction. When the main mirror is not pointed in the proper direction the reflected solar image is directed away from the detector and this condition is sensed. The solar/receiver image detector is positioned adjacent the projection screen along the line from the solar energy receiver on the tower through the mirror aperture. The mirror is positioned such that the image of the sun on the projection screen, when reflected by the limited aperture reflector is coincident with and/or concentric with the infrared image of the solar energy receiver at the solar/receiver image detector. Off-centering of the solar image with respect to the IR receiver image is detected at the detector and the gimbal mounting for the mirror is driven so as to bring the reflected image of the sun into coincidence with the image of the solar energy receiver.
In addition to the stability of the subject system against wind loading and the automatic mirror alignment without computer control, it is an important part of this invention that rather than utilizing a visual representation of the solar energy receiver, infrared energy radiated by the receiver is utilized in the mirror alignment process. It is also a important feature of this invention that rather than providing the detector with a lens, on one embodiment this focusing means is completely eliminated by virtue of the limited aperture reflector on the back of the mirror and in another embodiment by the use of a retro-reflecting coating on the solar image surface.
In one embodiment, the solar/receiver image detector is moved to compensate for sway of the solar energy receiver on the tower, thereby keeping the solar/receiver image detector positioned along the line from the instantaneous position of the solar energy receiver through the aperture of the mirror. It will be appreciated that with towers 300 feet tall or more, the sway of the receiver at the top of the tower may be as great as several feet due to high wind loading. With sways of this magnitude, the majority of the redirected solar radiation can miss the solar receiver which may only be one foot in diameter. The subject system corrects for the sway of the power tower by repositioning the solar/receiver image detector, and this is accomplished without the necessity of sensing the sway of the power tower at the tower itself. In one embodiment the movement or repositioning of the detector is accomplished by an XY servo drive system.
In another embodiment, a self-aligning mechanism for the detectors contemplates mounting the entire detector system on an array of bimetallic elements. These bimetallic elements cause the entire detector to sway in a direction opposite to the sway of the solar energy receiver, therefore in the same direction as its image, due to differential heating of the elements, thereby keeping the detectors aligned with the mirror aperture and receiver. Thus no complicated feedback loop mechanisms are necessary in the repositioning of the detector assembly to accommodate tower sway.
In a still further embodiment, the reflector on the back side of the mirror need not be limited in aperture if a lens is provided in the path between the solar image surface and the solar/receiver image detector. In this embodiment, one of the things which distinguishes the subject system from the conventional emergency mirror flashing system is that infrared or heat sensors are utilized to track the position of the solar energy receiver. Since the solar energy receiver is typically operated at a temperature between 1500.degree. F and 2500.degree. F, there is sufficient infrared radiation to be detected at large distances from the solar receiver. While this detector also simultaneously detects the position of the sun relative to the mirror it does this in the visible region of the electromagnetic spectrum. On the other hand infrared electromagnetic energy from the solar energy receiver defines the other parameter for proper mirror pointing. The advantage of this system over the detection system which do not differentiate between the images is that coatings can be applied, for instance to the reflector on the back of the mirror or to the projection screen, to provide a pass band for light in the visible region of the electromagnetic spectrum while rejecting infrared radiation. This permits discrimination between the image of the sun and the image of the solar receiver. Likewise the various portions of the detector can be provided with color selective coatings, thereby to enhance the discrimination between the two images.
Discrimination between the two images may be important when, for instance, the infrared radiation from the solar receiver is sharply focused, while radiation representing the sun's image covers a much broader area. This is the case with the use of the aforementioned limited aperture reflector on the back of the main mirror which spreads out the sun's image, and permits the use of concentrically mounted detectors. In this case the infrared portion of the detector is centrally located and is surrounded by conventional photocells for detecting the less concentrated sun's image. In both cases quadrant type detectors involving the use of thermal-sensitive and photo-sensitive elements respectively may be utilized.
As to the positioning of the mirror itself, this is accomplished by the use of motors which drive it around `X` and `Y` axes, in accordance with the output of differential amplifiers having inputs connected to opposing quadrant members of the reflected solar image detector.
It is therefore an object of this invention to provide an improved method and apparatus for automatically steering individual mirrors in a mirror field, in which there are no external parts in front of the mirrors, where these parts are protected from exposure, and wherein the system is stable, reliable, inexpensive to make and maintain, rugged and posses inherent alignment precision.
It is another object of this invention to provide a unique solar mirror drive system for mirrors in a mirror field in which no parts supported by the mirror present any appreciable lever arm, thereby contributing to the mechanical stability of the system.
It is still a further object of this invention to provide a mirror pointing system in which images of the sun and the receiver are produced in back of the mirror with an imaging surface in the form of a semi-transparent projection screen located in back of the mirror onto which the sun is imaged.
It is another object of this invention to provide a mirror pointing system in which movement of the distant object onto which solar energy is to be directed is accommodated by movement of detection apparatus in back of the mirror.
It is a still further object of this invention to provide a mirror pointing system in which two different types of radiation are utilized in the formation of concentric images for mirror alignment detection.
These and other objects of the inventions will be better understood where in connection the following specification in conjunction with the following drawings wherein